The present invention relates in general to estimating error rates in digital communication systems and in particular to estimating error rates using the filtered output of a predetermined pattern that has been delayed, inverted and exclusive-ored with itself.
Wireless communication systems are susceptible to a wide variety of interference sources. As a result, various schemes to encode error correction data and estimate error rates are commonly employed. By estimating the current error rate associated with a transmission, systems can adaptively adjust relevant equalization systems in order to optimize performance.
Typically systems will transmit a known test pattern and determine what portion of the known test pattern is in error at the receiving end. However, in systems employing a regenerative relay device, such as a satellite with on-board remodulation and demodulation, the estimate is an end-to-end estimate. These ground-based measurements are inadequate because it is difficult to separate user-to-relay link performance from variations in the space-to-ground link. Current systems may transmit a known test sequence that allows tuning ground-based equalizers, but the error rate is typically measured using large, power-consumptive test equipment.
In accordance with a first aspect of the invention, an apparatus for estimating an error rate in a digital communication system is provided. The apparatus comprises a demodulator for converting a received signal at a first time into a first sample and for converting the received signal at a second time into a second sample. The first sample includes a first bit digitally representing the received signal at the first time, and the second sample includes a second bit digitally representing the received signal at the second time. Further, a delay unit is used to store the first bit. Still further, an inverter is used to invert the state of the bit. The apparatus further includes an exclusive-or unit for determining an exclusive-or result based on the second bit and the first bit after the first bit is delayed by the delay unit and inverted by the inverter. An averaging unit is employed to determine a moving average based in part on the exclusive-or result. The apparatus converts the moving average to a digital error rate estimation using an analog-to-digital converter.
The digital communication system may be a satellite system, and the demodulator may be a phase shift key demodulator. The demodulator may also be an amplitude demodulator, or may employ adaptive equalization. The first bit may be the least significant bit of the first sample, and the second bit may be the least significant bit of the second sample. In any of the foregoing, the averaging unit may comprise a low pass filter. An amplifier may be provided for increasing the level of the moving average prior to conversion by the analog-to-digital converter. In such an instance, the amplifier may be a logarithmic amplifier.
In accordance with a further aspect of the invention, a method for estimating an error rate in a digital communication system is provided. The method begins by converting a received signal at a first time into a first sample. The first sample includes a first bit digitally representing the received signal at the first time. The first bit is stored and inverted, and the received signal is converted at a second time into a second sample. The second sample includes a second bit digitally representing the received signal at the second time. An exclusive-or result is determined based on the first and second bits after the first bit is delayed by the delay unit and inverted by the inverter. A moving average is determined based in part on the exclusive-or result. The moving average is converted to a digital error rate estimation.
The digital communication system described in the preceding paragraph may be implemented as a satellite communications system. The received signal may be a square wave, and the step of converting the received signal into first and second samples includes demodulating the signal using a phase shift key demodulator. In such an instance, the phase shift key demodulator may comprise an eight phase shift key demodulator.
The method may also include adaptive equalization step. The first bit may be the least significant bit of the first sample and the second bit may be the least significant bit of the second sample. The step of determining a moving average may include low pass filtering. The method may also include a step whereby the level of the moving average is amplified. In such an instance, the step of amplifying may be implemented using a logarithmic algorithm.